Numerous industries employ processes that require accurate delivery of gas mixtures comprising a gas of interest dispersed within a carrier gas. Accurate delivery of such gas mixtures requires precise measurements of the concentration of the gas of interest in the flowing gas mixture, where the gas of interest is typically of high purity and may be highly corrosive. Examples of these processes include chemical vapor deposition (CVD), dopant diffusion (e.g., as practiced in the semiconductor industry), and operation of high efficiency hydrogen cooled generators.
One method of controlling the flux of the gas of interest or reactant is the use of a carrier gas, typically hydrogen or nitrogen, which is flowed through a vaporizer or “bubbler”. The flow of carrier gas is controlled by a mass flow controller, and the concentration of reactant in the gas stream as it exits the vaporizer is assumed to be constant, and the flux of the reactant is proportional to the flow of carrier gas. This approach is inaccurate for several reasons, including, variations in the bubbler temperature, instability of the temperature and pressure of the binary gas mixture, possible leakages in the gas lines upstream and downstream of the bubbler, and concentration time delays between the mass flow controllers and the points of interest, especially at low flow rates.
U.S. Pat. Nos. 6,116,080, 6,192,739, 6,199,423 and 6,279,379, commonly owned by the owner of this patent application, discloses a technique and device that is an improvement over the pre-mixing measurements. These patents disclose an acoustical measuring device that infers the concentration of a gas of interest downstream of the bubbler and after the vaporization process, herein referred to as a “post-mixing measurement.”
Certain aspects of these patents and the post-mixing measurement technique are embodied in the PIEZOCON Concentration Sensor (hereinafter “Piezocon Sensor”), manufactured and sold by Veeco Flow Technologies, Inc. of Poughkeepsie, N.Y., USA. The Piezocon Sensor utilizes an acoustical transformer comprising a low impedance interface. Herein, an “acoustical transformer” is defined as a layer or multi-layer interface between an acoustical element (sensor or driver) such as a piezoelement and the medium under measurement. Desirable characteristics of acoustical transformers include high efficiency over a broad band of acoustical frequencies, matching of the low acoustical impedance of the test medium and an exposed surface having resistance to chemical reaction with the test medium. Polyimides, such as Kapton® film, are a preferred material for acoustical transformers because polyimides provide a low impedance matching layer having resistance to chemical reaction comparable to other materials traditionally used in acoustical transformers such as fluoropolymers while providing a more stable Young's Modulus across the temperatures of interest.
However, despite the reasonably high chemical resistance of polyimides, it has been discovered that, over time in a metal-organic chemical vapor deposition (MOCVD) system utilizing indium-gallium-nitride, the polyimide components of the acoustical transformers become coated with gallium and indium oxides, the accrual of which is believed to cause drift in the sensor due to reducing distance between the transducers and affecting the transfer function of the acoustical transformer. Polyimides are also known to swell due to absorption of the chemicals, which can also reduce the distance between the transducers and affect the transfer function.
There is a need, therefore, for an acoustical transformer for use in post-mixing measurements that possess the favorable mechanical attributes of polyimide while mitigating the attendant chemical reactions that occur in certain post-mixing measurement environments.